Zero turn radius mowers (also referred to as zero turn mowers) are speedy and low to the ground. The name “zero turn” refers to the ability of this type mower to turn on a very short radius. Zero turn type mowers have improved productivity for landscapers, grounds maintenance and commercial lawn care crews. However, zero turn type mowers also have hidden dangers when used on steep terrain. Riding a zero turn mower in areas with water, retaining wall drop-offs and slopes present a challenge due to the zero turn mower's center of gravity which can be relatively high when in operation and particularly when carrying a tall and heavy operator all of which can expose the operator to a strong possibility of rollover and serious personal injury. Since a zero turn mower is both highly maneuverable, and exceptionally fast it allows the operator to get the job done quickly. Therefore, operators of zero turn mowers tend to speed along at a high speed without realizing the risks involved. While many of the zero turn mower-related injuries that occur are the direct result of excessive speed, operation of a zero turn mower on sloped terrain in itself presents a challenge especially because of the nature of the front wheels.
The conventional zero turn mower currently available on the market has a set of caster wheels on the front and a pair of engine driven rear drive wheels on the back. Each rear wheel is independently driven and is independently controlled as to direction by means of an asymmetrical drive means between the engine and the rear drive wheel. By braking one rear wheel and driving the other rear wheel, the operator is able to make the mower rotate about a vertical axis, about which the rear drive wheels rotate. A serious problem arises when the conventional type of zero turn mower is driven transverse to a slope in that the mower tends to rotate somewhat as if it were making a zero turn and causes the operator to lose control and the mower to slip downhill. It is believed that this tendency to slip derives from the fact that the front caster-type wheels do not provide any steering input and therefore can, on a sloped terrain, permit the zero turn mower to point down slope and thereby risk going over an embankment or flipping over.
Many walk behind mowers also employ caster wheels and experience similar problems when operated on sloped terrain. The tendency of the caster wheels to point downwards on slopes permits a zero turn or walk behind type mower so equipped to lose traction and be difficult to maintain in a straight line.
As useful background, U.S. Pat. No. 4,735,038 illustrates a prior attempt to make a hand pushed powered lawnmower safer when on an embankment. U.S. Pat. No. 4,707,971 illustrates a prior attempt to make a riding type mower safer when on a slope. With regard to use of sprocket chain drives in the device of the present invention, note is made of U.S. Pat. No. 6,601,665 which illustrates a utility vehicle with one pair of midway disposed wheels driven through sprocket-chain drives by another pair of rearwardly disposed wheels, said pair of midway disposed wheels having the ability to change position relative to one another in correspondence with the nature of the terrain. Also felt to be useful as background is a relatively recent comparison of various kinds of zero turn mowers which is to be found in the Popular Mechanics article by Roy Berendsohn, entitled, “Taking Turns”, published on Jun. 1, 2000. A discussion of the hill slipping problem dealt with by the present invention is discussed in the article “My Zero Twin Scag Tiger Cub is Safer on My Hill” published Sep. 24, 2003 by Epinions.com on its website http://www.epionions.com in its Home and Garden Section.
With the above background in mind, the device of the invention provides a mower with a set of front caster wheels and a set of rear driven wheels with auxiliary right and left wheels rotatably mounted on the rearward end of a pair of spaced apart rigid members which pivot on and extend rearward from the rear of the mower. Each auxiliary wheel of the device is driven, in the illustrated embodiments, by its own chain drive by a respective primary right or left drive wheel of the mower.
The device of the invention can either be incorporated in original mower equipment or be incorporated as an add-on to mowers with front caster-type wheels and a pair of driven rear wheels. The mowers may be either zero turn type mowers or walk behind type mowers. The device of the invention, in effect, stabilizes traction, especially on relatively steep sloped terrain by minimizing the tendency of the mower to slip when linearly traversing across a relatively steep sloped surface. The device of the invention furthermore provides for both increased friction between the mower's rear wheels and the ground as well as an auxiliary drive force whose inertia is asserted on the mower in a forward direction. The increased friction and drive force tend to minimize the tendency of the mower to turn downwards on a relatively steep slope.